The present invention relates to a technology used when forming image(s), in for example printers or plane paper copying machines (xe2x80x9cPPC""sxe2x80x9d), with an improvement in the driving of a developing unit.
Examples of prior art of this type of technology are as follows. A method of and an apparatus for forming image has been disclosed in Japanese Patent Application Laid-Open No. 10-177286, an image formation apparatus has been disclosed in Japanese Patent Application Laid-Open No. 11-109708, and a method of and an apparatus for forming image has been disclosed in Japanese Patent Application Laid-Open No. 11-125968. According to the structures described in these publications, two image stations are disposed around an intermediate transfer unit. Two developing units are disposed in each photo conductor provided in each image station. These two developing units are changed over between them to form visible images of different colors on the photo conductor. Visible images prepared on the two photo conductors respectively are sequentially transferred in superimposition onto the intermediate transfer unit. A visible image obtained based on the superimposed transfer is transferred again onto a paper to form a color image.
Japanese Patent Application Laid-Open No. 10-177286 points out various problems that the image formation apparatus becomes large, that the number of printed sheets per unit time is small, and that the apparatus requires many image processing units.
In order to solve these problems, the invention disclosed in this publication has the following structure. The image formation apparatus has an intermediate transfer belt onto which a toner image on a photo conductor drum is transferred, and a transfer unit that transfers a color image on the intermediate transfer belt onto a transfer paper. In the above structure, first and second image formation units are disposed, with a distance between them, along the intermediate transfer belt. On the first image formation unit, there are provided one photo conductor drum, a developing unit that develops an electrostatic latent image on the photo conductor drum with a toner of color A, and a developing unit that develops this electrostatic latent image with a toner of color B. On the second image formation unit, there are provided one photo conductor drum, a developing unit that develops an electrostatic latent image on the photo conductor drum with a toner of color C, and a developing unit that develops this electrostatic latent image with a toner of black color.
According to Japanese Patent Application Laid-Open No. 11-109708, two developing units are disposed on each photo conductor provided in each of two image stations that are disposed around the intermediate transfer unit. These two developing units are changed over between them to form visible images of different colors on the photo conductor. Visible images prepared on the two photo conductors respectively are sequentially transferred in superimposition onto the intermediate transfer unit. A visible image obtained based on the superimposed transfer is transferred again onto a paper to form a color image. It is an object of this invention to provide a changeover unit that changes over between the developing units in this image formation apparatus.
In order to meet this object, the invention of this publication provides one development driving system that drives the two developing units, and a changeover unit that changes over the power of the development driving system to anyone of the two developing units, in each image station.
Japanese Patent Application Laid-Open No. 11-125968 describes a structure that two developing units are disposed adjacently which face the external periphery of the image holder. One developing unit develops a latent image on the image holder into a visible image of an optional color. The other developing unit develops the same latent image on the same image holder into a visible image of a color different from the color used by the former developing unit. According to this structure, the developing units that develop the latent images into visible images are changed over between them during the rotation of the image holder, thereby to change over colors. The invention of this publication has an object of providing margin in the developing unit changeover time at the time of sequentially developing the latent images into visible images using two colors.
In order to meet this object, according to the invention of this publication, a developing unit positioned at an upstream in the rotation direction of the image holder (an upstream developing unit) first starts development out of the two developing units. A developing unit positioned at a downstream (a downstream developing unit) starts development next.
According to the image formation method disclosed in the above publication, changing over a developing unit to be used for carrying out development from the downstream developing unit to the upstream developing unit (hereinafter, to be referred to as a changeover of a developing function) has the following problem. A time taken from when the end of an image that has been first developed into a visible image has passed through a developing unit that has carried out this development till when the start of the next latent image to be developed into a visible image by the other developing unit reaches the developing unit that is to develop this latent image, becomes shorter than the time taken when the developing unit is changed over from the upstream developing unit to the downstream developing unit.
Therefore, according to the invention described in Japanese Patent Application Laid-Open No. 11-125968, after the upstream developing unit has first carried out the image formation, the developing function is changed over to downstream the developing unit. Based on this, margin is provided in the time of changing over the developing function.
However, the above conventional technique has had the following problems.
Often, the image formation apparatus outputs a plurality of images continuously. Therefore, a high-speed printing has been desired for this continuous output. For carrying out a continuous output, it is necessary to alternately execute the changeover of the developing function from the upstream developing unit to the downstream developing unit, and the changeover of the developing function from the downstream developing unit to the upstream developing unit. It is necessary to execute this change over continuously.
Therefore, it is necessary to accommodate the time required for the changeover of the developing function within a shorter period of time out of the developing function changeover times in both directions. It is difficult to provide margin in the developing function changeover times. When it is not possible to accommodate the developing function changeover time within a shorter changeover time, it is necessary to increase the peripheral length of the intermediate transfer belt. However, increasing the peripheral length of the intermediate transfer belt becomes a large hindrance to the increasing of the print speed, and reducing the sizes and lowering the cost of the image formation apparatus.
It is an object of the present invention to provide an image formation apparatus and an image formation method capable of realizing a high-speed printing, and a size reduction and cost reduction of the apparatus.
The image formation apparatus according to one aspect of the present invention comprises an image holder, a first developing unit, and a second developing unit, wherein a latent images on the image holder is developed by changing over between the first developing unit and the second developing unit. Out of the first developing unit and the second developing unit, a changeover time T2 to change over from a developing unit positioned at a downstream in a rotation direction to a developing unit positioned at an upstream in the rotation direction of the image holder is set shorter than a changeover time T1 to change over a developing unit from the developing unit positioned at an upstream in the rotation direction to the developing unit positioned at a downstream in the rotation direction.
The image formation method executed on an image formation apparatus that comprises a first developing unit and a second developing unit, and that develops latent images on an image holder using the first developing unit and the second developing unit. The image formation method comprises developing a latent image on the image holder using a developing unit positioned at an upstream in the rotation direction of the image holder, out of the first developing unit and the second developing unit, changing over a developing unit used for development from a developing unit positioned at an upstream in the rotation direction of the image holder to a developing unit positioned at a downstream in the rotation direction of the image holder over a changeover time T1, developing a latent image on the image holder using the developing unit positioned at a downstream in the rotation direction of the image holder, and changing over a developing unit used for development from the developing unit positioned at a downstream in the rotation direction of the image holder to the developing unit positioned at an upstream in the rotation direction of the image holder over a changeover time T2 that is shorter than the changeover time T1.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.